This proposal is based on my previous observations which indicate that; 1) rat brain contains an endogenous inhibitor of UDP-galactose:ceramide galactosyltransferase; 2) that lipid within the membrane play a role in the expression of the activity of UDP-galactose:ceramide galactosyltransferase, and 3) the rat myelin sheath, as well as isolated axolemma-enriched fractions, contains UDP-galactose:ceramide galactosyltransferase. The first series of experiments is is designed to determine if the endogenous inhibitor of ceramide galactosyltransferase has any physiological role in the regulation of the synthesis of galactosylceramides. Studies on purification, characterization and developmental changes of the endogenous inhibitor will be carried out. Attempts will also be made to obtain definitive answer on the possible ientity of UDP-galactose-sphingosine galactosyltransfrase, UDP-galactose:NFA ceramide galactosylransferase and UDP-galactose:HFA ceramide galactosyltransferase. The second series of experiments involves solubilization, purification and characterization of rat brain UDP-galactose:ceramide galactosyltransferase. Particular attention will be focused on the possibility that the enzyme may be lipid-requiring and on the roleof lipids in the expression of the enzyme activity. Attempts will be made to conclusively determine if the galactosyltransferase in myelin an axolemma-enriched fractions are different from that in microsomes. The long-term aim of this project is to understand how the synthesis of galactosylceramide, a major component of myelin, is regulated in vivo.